https://leetcode.cn/problems/er-cha-shu-de-zui-jin-gong-gong-zu-xian-lcof/description/

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class Solution {
public:
    TreeNode* lowestCommonAncestor(TreeNode* root, TreeNode* p, TreeNode* q) 
    {
        if(root == nullptr || p == root || q == root)
        {
            return root;
        }

        TreeNode *l = lowestCommonAncestor(root->left, p, q);
        TreeNode *r = lowestCommonAncestor(root->right, p, q);

        return l == nullptr ? r : (r == nullptr ? l : root);
    }
};